Locked loops, bias generators, charge pumps and methods for generating control voltages

ABSTRACT

Locked loops, bias generators, charge pumps and methods for generating control voltages are disclosed, such as a bias generator that generates bias voltages for use by a clock signal generator, such as a voltage controlled delay line, in a locked loop having a phase detector and a charge pump. The charge pump can either charge or discharge a capacitor as a function of a signal from the phase detector to generate a control voltage. The bias generator can receive the control voltage from the capacitor, and it generates bias voltages corresponding thereto. A portion of the bias generator can have a topography that is substantially the same as at least a portion of the topography of the charge pump. As a result, it can cause the charge pump to charge the capacitor at the same rate that it discharges the capacitor over a relatively wide range of control voltages. The charge pump and the bias generator can also include circuitry for limiting the charging of the capacitor when the control voltage is relatively low.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/355,523, filed Jan. 16, 2009, U.S. Pat. No. 7,812,652. Thisapplication and patent is incorporated by reference herein in itsentirety and for all purposes.

TECHNICAL FIELD

This invention relates to semiconductor integrated circuits, and, moreparticularly, in one or more embodiments, to bias generators, chargepumps and methods for generating control voltages to voltage controlleddelay lines and voltage controlled oscillators in locked loop circuits.

BACKGROUND OF THE INVENTION

A number of different types of locked loop circuits are used inconventional integrated circuits, the two most notable being delay lockloops and phase lock loops. Both of these types of locked loops use aphase detector to compare the phase of a reference clock signal to thephase of a feedback clock signal generated by the locked loop. A phaseerror signal generated from the comparison is applied to a controller(i.e., a combination of a charge pump and a bias generator) which, inturn, generates an appropriate control signal(s) that is applied to avariable delay line in the case of a delay lock loop or a voltagecontrolled oscillator in the case of a phase lock loop.

A typical prior art delay lock loop 10 is shown in FIG. 1. The delaylock loop 10 includes a phase detector 12 having a first input receivinga reference clock signal Clk_ref and a second input receiving a feedbackclock signal Clk_fb, which is generated from an output clock signalClk_out. The phase detector 12 generates an UP signal in response to aphase error in one direction, and it generates a DN signal in responseto a phase error in the opposite direction. These UP and DN signals areapplied to a charge pump 16, which provides a control voltage Vct acrossa capacitance, such as capacitor 18. As explained in greater detailbelow, the charge pump 16 also receives a feedback voltage Vfb, whichattempts to maintain the rate of charge of the capacitor 18 equal to therate of discharge. In response to the UP signal, the charge pump 16charges the capacitor 18 to increase the control voltage Vct, and, inresponse to the DN signal, the charge pump 16 discharges the capacitor18 to decrease the control voltage Vct.

The control voltage Vct is applied to a bias generator 20 that generatestwo bias voltages Vbp, Vbn as a function of the magnitude of the controlvoltage Vct. These bias voltages Vbp, Vbn control the delay of a voltagecontrolled delay line 24 as the reference clock signal Clk_ref iscoupled through the delay line 24 to generate the output clock signalClk_out.

There is therefore a need for an improved bias generator operating witha charge pump in a locked loop, such as one that ensures a more evenbalance between the charge current and the discharge current of thecharge pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art delay lock loop that can useembodiments of a charge pump and bias generator in accordance with theinvention.

FIG. 2 is a schematic diagram of a typical prior art bias generator.

FIG. 3 is a schematic diagram of a bias generator according to anembodiment of the invention.

FIG. 4 is a schematic diagram of a charge pump according to anembodiment of the invention.

FIG. 5 is a schematic diagram of a delay stage that can be used in thevoltage controlled delay line that is used in the delay lock loop ofFIG. 1.

DETAILED DESCRIPTION

The charge pump 16 used in the prior art delay lock loop 10 is able tobalance the charge and discharge currents to only a limited extent. Onereason for this limitation is that the topography of the charge pump 16differs too greatly from the topography of the bias generator 20 and thevoltage controlled delay line 24, which results in an excessive lack ofoperating symmetry between the charge pump 16 on the one hand and thebias generator 20 and the voltage controlled delay line 24 on the other.Another reason is that, when the control voltage Vct is at the low endof its operating range, the relatively large voltage differentialbetween the supply voltage Vcc and the control voltage Vct results in anexcessive charging current. For example, with a supply voltage Vcc of 1volt, the operating range of Vct is between 0 volts and 0.5 volts. Yetthe voltage differential between Vcc and the control voltage Vct is sogreat at the low end of the range that the discharge and charge currentsare substantially equal only about in the range 0.35 volt to 0.5 volt.

A typical example of a prior art bias generator 30 that can be used asthe bias generator 20 (FIG. 1) is shown in FIG. 2. The bias generator 30includes a differential pair of PMOS transistors 32, 34, of which 32receives the control voltage Vct at its gate. The gate of the other PMOStransistor 34 is coupled to the drain of the transistor 32 in a diodeconfiguration. These sources of both transistors 32, 34 are coupled to asupply voltage Vcc. The combined current flowing through the transistors32, 34 is controlled by a first NMOS transistor 36, which is biased to aconductive state, and a second NMOS transistor 38 as a function of avoltage applied to its gate. A high gain differential amplifier 40receives at respective inputs the control voltage Vct and a feedbackvoltage from the drains of the PMOS transistors 32, 34 and generates oneof the bias voltages Vbn. This bias voltage Vbn is applied to the gateof the transistor 38 to control the combined current flowing through thePMOS transistors 32, 34.

The bias generator 30 also includes a second pair of differential PMOStransistors 50, 52, both of which have their gates diode-coupled. Thecombined current through the transistors 50, 52 is also controlled by anNMOS transistor 56 having its gate coupled to the supply voltage Vcc anda second NMOS transistor 58, which also receives the bias voltage Vbn atits gate. The other bias voltage Vbp is generated at the drains of thetransistors 50, 52.

In operation, a decrease in the control voltage Vct correspondinglydecreases the voltage drop across the transistors 32, 34 therebyincreasing the feedback voltage applied to the positive input of thedifferential amplifier 40. As a result, the bias voltage Vbn generatedby the differential amplifier 40 increases, thereby increasing thecurrent flowing through the NMOS transistor 38. This increased currentreduces the feedback voltage applied to the differential amplifier 40until it is substantially equal to the control voltage Vct. The circuitresponds in the opposite manner to an increase in the control voltageVct to decrease the bias voltage Vbn. Thus, the bias voltage Vbn variesinversely with the control voltage Vct.

An increase in the bias voltage Vbn also increases the current flowingthrough the PMOS transistors 50, 52, thereby decreasing the magnitude ofthe bias voltage Vbp. Thus, the bias voltage Vbp varies inversely withthe bias voltage Vbn and, therefore, in the same manner as the controlvoltage Vct. Insofar as the circuit generating the bias voltage Vbp issubstantially to the same as the circuit generating the feedback voltageapplied to the differential amplifier 40, the magnitude of the biasvoltage Vbp is substantially equal to the feedback voltage. Further,since the amplifier 40 has a very high gain, the magnitude of thefeedback voltage Vbp is substantially equal to the magnitude of thecontrol voltage Vct. Therefore, the magnitude of the bias voltage Vbp issubstantially equal to the magnitude of the control voltage Vct.

As will be explained below, the topography of the prior art biasgenerator 30 differs substantially from the topography of the chargepump 16 that is typically used. In fact, as will be explained below, thebias generator 30 uses a topography that is very similar to thetopography used in a delay stage of the voltage controlled delay line24. The charge pump 16 typically used is thus generally unable toclosely balance the charge current and the discharge current of thecapacitor 18.

A bias generator 60 according to an embodiment of the invention is shownin FIG. 3. The bias generator 60, like the bias generator 30, includesthe differential amplifier 40 having inputs receiving the controlvoltage Vct and a feedback voltage, and generating one of the biasvoltages Vbn at the output of the amplifier 40. The bias voltage Vbn isapplied to the gate of an NMOS transistor 64, which is connected inseries with a resistance, such as a resistor 66, between the supplyvoltage Vcc and ground.

In operation, an increase in the bias voltage Vbn increases the currentflowing through the resistor 66, thereby increasing the magnitude of thesecond bias voltage Vbn2. The bias generator 60 also includes two PMOStransistors 70, 72 connected in series with the supply voltage Vcc,although the PMOS transistor 70 may be omitted in some embodiments ofthe invention. The PMOS transistor 70 receives the second bias voltageVbn2 at its gate, and the PMOS transistor 72 receives the controlvoltage Vct at its gate. Two additional PMOS transistors 76, 78 and 2NMOS transistors 80, 82 are coupled in a parallel/series configuration.The PMOS transistors 76, 78 are biased to a conductive state by havingtheir gates coupled to ground, and the NMOS transistors 80, 82 arelikewise biased to a conductive state by having their gates coupled tothe supply voltage Vcc. Finally, the parallel/series combination oftransistors are coupled to ground through an NMOS transistor 86, whichreceives the bias voltage Vbn at its gate.

In operation, an increase in the control voltage Vct results in adecrease in the bias voltage Vbn at the output of the differentialamplifier 40. This decreased voltage of Vbn also reduces the magnitudeof the bias voltage Vbn2. The decreased voltage Vbn results in anincrease in the voltage at the drain of the NMOS transistor 86, therebyresulting in an increase in the voltage fed back to the positive inputof the differential amplifier 40 until the magnitude of the feedbackvoltage is substantially equal to the magnitude of the control voltageVct. The corresponding decrease in the voltage Vbn2 would have atendency to increase the feedback voltage. However, this tendency iscountered to some extent by the increase in the control voltage Vctapplied to the gate of the PMOS transistors 72. At any rate, thistendency is not enough to have an effect on the feedback voltage that isequal to the effect of the decrease in the voltage Vbn applied to thegate of the transistor 86. Thus, the magnitudes of the bias voltages Vbnand Vbn2 vary inversely with the magnitude of the control voltage Vct.Also, by coupling the gates of the PMOS transistors 76, 78 to ground andcoupling the gates of the NMOS transistors 80, 82 to Vcc, the feedbackvoltage is compensated for variations in the supply voltage Vcc.

A charge pump 120 according to one embodiment of the invention is shownin FIG. 4. As explained in greater detail, the circuitry used in thecharge pump 120 can be balanced to provide the control voltage Vct in amanner that does not alter the current drawn by the charge pump 120. Thecharge pump 120 includes a parallel/series combination of PMOStransistors 124, 126 and NMOS transistors 130, 132. The control voltageVct is produced at the junctions between the PMOS transistors 124, 126and the NMOS transistors 130, 132. The gate of the NMOS transistor 130receives the UP signal from the phase detector 12 (FIG. 1) while thegate of the PMOS transistor 124 receives a complementary UP_signal.Similarly, the gate of the NMOS transistor 132 receives the DN signalfrom the phase detector 12 while the gate of the PMOS transistor 126receives a complementary DN_signal.

In operation, an increase in the UP signal and a corresponding decreasein the UP_signal results in an increase in the control voltage Vct, andan increase in the DN signal and a corresponding decrease in theDN_signal results in a decrease in the control voltage Vct. However, thechanges in current resulting from these signals is balanced by othertransistors in the charge pump 120. Specifically, PMOS transistors 134,136 have substantially the same topography as the PMOS transistors 124,126, but they receive signals that are the complement of the signalsreceived by the PMOS transistors 124, 126. Similarly, the NMOStransistors 140, 142 have substantially the same topography as the NMOStransistors 130, 132, but they receive signals that are the complementof the signals received by the NMOS transistors 130, 132.

The transistors 124-142 are also balanced by the remaining transistorsin a similar configuration in the charge pump 120. Specifically, PMOStransistors 144, 146 have substantially the same topography and receivethe same signals as the PMOS transistors 124, 126, and NMOS transistors150, 152 have substantially the same topography and receive the samesignals as the NMOS transistors 130, 132. Similarly, a PMOS transistor154 and an NMOS transistor 160 have substantially the same topographyand receive the same signals as the PMOS transistor 134 and the NMOStransistor 140, respectively. While a PMOS transistor 166 and an NMOStransistor 172 have substantially the same topography as the PMOStransistor 136 and the NMOS transistor 142, they do not receive the samesignals. However, a PMOS transistor 176 and an NMOS transistor 182 havesubstantially the same topography and receive the same signals as thePMOS transistor 136 and the NMOS transistor 142, respectively. Also,PMOS transistor 186 and NMOS transistor 192 have substantially the sametopography and receive the same signals as the PMOS transistor 166 andthe NMOS transistor 172, respectively. Therefore, the circuit operatesin a balanced manner.

The magnitude of the current through the above-described transistors arecontrolled by NMOS transistors 200, 202, 204 and PMOS transistors 210,212, 214, 216, 218 and 220. The PMOS transistors 212, 216, 220 arecontrolled by a feedback signal generated at the junctions between theNMOS transistors 134, 136 and the PMOS transistors 140, 142 as well asthe junctions of the PMOS transistors 144, 146 and the NMOS transistors150, 152. Because of the symmetry of the circuit, the feedback voltageis substantially equal to the control voltage Vct. In operation, thebias voltage Vbn fed back to the gates of the NMOS transistors 200-204result in negative feedback since, as explained above, the bias voltageVbn varies inversely with the control voltage Vct. The bias voltage Vbnfunctions to maintain the charge rate and discharge rate of thecapacitor 18 balanced.

At least some conventional charge pumps are similar to the charge pump120 shown in FIG. 4. The charge pump 120 differs from these conventionalcharge pumps by including the PMOS transistors 210-218 in series witheach of the PMOS transistors 212-220, respectively. As explained above,the bias voltage Vbn2 varies inversely with the control voltage Vct.Therefore, when the control voltage Vct is at the bottom of itsoperating range, the bias voltage Vbn2 applied to the gates of the PMOStransistors 210, 214, 218 increases to reduce the magnitude of thecurrent charging the capacitor 18, thereby preventing the capacitor 18from being charged at a faster rate than it is discharged. As a result,the charge pump 120 provides a substantially more balanced charging anddischarging of the capacitor 18.

Although the use of the PMOS transistors 210, 214, 218 provides theadvantage of preventing more current from being sourced to the capacitor18 than can be sunk by the NMOS transistors 200-204, they may be omittedin some embodiments of the invention. The charge pump 120 will stillprovide more balanced charging and discharging of the capacitor 18 aslong as it is used with a bias generator, like the bias generator 60,that has substantially the same topography as the charge pump 120. Thus,if the PMOS transistors 210, 214, 218 are included in an embodiment ofthe charge pump, the PMOS transistor 70 (FIG. 3) should ideally beincluded in an embodiment of the bias generator. As explained above,prior art bias generators have a topography that is substantiallydifferent than the topography used by the bias generator 60. Insofar asthe bias generator 60 has a topography that is substantially the same asat least a portion of the topography of the charge pump 120, thecharging and discharging balance of the charge pump 120 is substantiallyimproved.

A stage 240 of the delay line 24 that can be used in the delay lockedloop 10 of FIG. 1 when it contains the charge pump 120 and biasgenerator 60 is shown in FIG. 5. The delay stage 240 includes a firstpair of PMOS transistors 242, 244 and a second pair of PMOS transistors246, 248 all of which have their respective sources coupled to thesupply voltage Vcc. The drains of the PMOS transistors 242, 244 areconnected to each other, and the gate of the PMOS transistor 244 has adiode-coupled configuration. The gate of the PMOS transistor 242receives the bias voltage Vbp. Similarly, the drains of the PMOStransistors 246, 248 are connected to each other, and the gate of thePMOS transistor 248 has a diode-coupled configuration. The gate of afirst NMOS transistor 250 receives the reference clock signal Clk_refand the gate of a second NMOS transistor 254 receives a complement ofthe reference clock signal, Clk_ref_. Finally, the gate of an NMOStransistor 260 also receives the bias signal Vbn. Complementary clockoutput signals Outn, Outp are generated at the drains of the NMOStransistors 250, 254, respectively.

In operation, an increase in the bias voltage Vbp results in an increasein the time required to switch the clock output signals Outn, Outp high,and a corresponding decrease in the bias voltage Vbn results in anincrease in the time required to switch the output signals Outn, Outplow. Therefore, the delay of the voltage controlled delay line 24 usingthe delay stages 240 is increased. Conversely, a decrease in the biasvoltage Vbp results in a decrease in the time required to switch theclock output signals Outn, Outp high, and a corresponding increase inthe bias voltage Vbn results in a decrease in the time required toswitch the output signals Outn, Outp low. As a result, the delay of thevoltage controlled delay line 24 using the delay stages 240 isdecreased.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from theinvention. Such modifications are well within the skill of thoseordinarily skilled in the art. Accordingly, the invention is not limitedexcept as by the appended claims.

1. A method of generating a control voltage for a voltage controlleddelay line, comprising: receiving a first signal indicative of adifference in phase between a reference clock signal and an output clocksignal; generating at a first circuit a second signal from the firstsignal; and using the second signal to generate at a second circuit thecontrol voltage, wherein first and second circuits both include: a firsttransistor of a first type and a first transistor of a second typecoupled to each other in a first parallel combination; and a secondtransistor of the first type and a second transistor of the second typecoupled to each other in a second parallel combination and in serieswith the first parallel combination.
 2. The method of claim 1 whereinthe control voltage comprises first and second control voltages, andwherein the act of using the second signal to generate at the secondcircuit the control voltage comprises: generating the first controlvoltage with a magnitude corresponding to a difference between amagnitude of a voltage of the second signal and a magnitude of afeedback voltage; generating the second control voltage from the firstcontrol voltage; and using the first and second control voltages togenerate the feedback voltage.
 3. The method of claim 2, furthercomprising compensating the feedback voltage for variations in at leastone supply voltage used to generate the first and second controlvoltages.
 4. The method of claim 2 further comprising: increasing a timeto switch an output of a stage of the voltage controlled delay line highin response to an increase in the first control voltage; and decreasingthe time to switch the output of the stage of the voltage controlleddelay line high in response to a decrease in the first control voltage.5. The method of claim 4 further comprising: increasing a time to switchthe output of the stage of the voltage controlled delay line low inresponse to a decrease in the second control voltage; and decreasing thetime to switch the output of the stage of the voltage controlled delayline low in response to an increase in the second control voltage. 6.The method of claim 3, further comprising providing a second signal in amanner that does not substantially alter a current drawn.
 7. The methodof claim 6, further comprising limiting a current of the second signalwhen a voltage of the second signal is at a lower limit of an operatingrange of the second signal.
 8. The method of claim 1, wherein the firstsignal is comprised of an UP signal and a DOWN signal, the methodfurther comprising: increasing the second signal in response to anincrease in the UP signal; and decreasing the second signal in responseto a decrease in the DOWN signal.
 9. The method of claim 1, furthercomprising generating the first signal at a phase detector.
 10. Themethod of claim 1 wherein the first circuit comprises a charge pump. 11.The method of claim 10 wherein the charge pump comprises: a thirdtransistor of the first type coupled between a first power supplyvoltage and the first parallel combination; and a third transistor ofthe second type coupled between a second power supply voltage and thesecond parallel combination.
 12. The method of claim 1 wherein thesecond circuit comprises a bias generator.
 13. A method of generating acontrol voltage for a voltage controlled delay line, comprising:comparing a phase of a reference clock and a phase of a feedback clockto provide a phase error signal indicative of a phase difference;generating a control signal at a charge pump responsive to the phaseerror signal, wherein the charge pump includes a first circuit having aparallel/series combination of transistors used in generating thecontrol signal, the combination of transistors including series coupledtransistors of a first type and series coupled transistors of a secondtype; generating a control voltage at a bias generator corresponding tothe control signal, wherein the bias generator includes a second circuithaving the same parallel/series combination of transistors used ingenerating the control voltage, the combination of transistors includingseries coupled transistors of the first type and series coupledtransistors of the second type; and providing the feedback clock forcomparison having a phase based at least in part on the control voltage.14. The method of claim 13 wherein generating a control signalresponsive the phase error signal comprises: charging and discharging acapacitor based at least in part on the phase error signal.
 15. Themethod of claim 13 wherein providing the feedback clock for comparisonhaving a phase based at least in part on the control voltage comprises:delaying the reference clock by a delay adjusted responsive to the biasvoltage to provide the feedback clock.
 16. The method of claim 13wherein comparing a phase of a reference clock and a phase of a feedbackclock to provide a phase error signal comprises: generating a firsterror signal responsive to a first polarity of the difference betweenthe phase of the reference clock and the phase of the feedback clock;and generating a second error signal responsive to a second polarity ofthe difference between the phase of the reference clock and the phase ofthe feedback clock.
 17. The method of claim 16 wherein generating acontrol signal responsive the phase error signal comprises: charging acapacitor responsive the first error signal; and discharging a capacitorresponsive the second error signal.
 18. The method of claim 13 whereingenerating a control voltage corresponding to the control signalcomprises: generating the control voltage using the bias generatorincluding a portion having a first topography; and generating thecontrol signal using the charge pump including a portion having a secondtopography, the second topography of the charge pump substantiallysimilar to the first topography of the bias generator.
 19. The method ofclaim 13, further comprising: compensating the feedback clock forvariations in at least one supply voltage used to generate the controlsignal.
 20. The method of claim 13 wherein generating the controlvoltage comprises: generating first and second control voltages.